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Sunday, 24 July 2011

Dealing with the cables

Every High-Band Array (HBA) tile antenna has two cables that connect it to the main KAIRA processing electronics. These electronics are located in the so-called RF-container to the north of the array. The two cables from each antenna carry the RF (= radio frequency) signal back to the RF-container. There is one cable for each polarisation. In addition to that, one cable carries a low-baud-rate command signal from the RF-container out to the antenna and the other carries DC power to energise the circuits in the antenna tiles themselves.

For most LOFAR stations, the cables are buried underground. This provides security against wildlife, but it also provides thermal stability. When a cable is heated up, its properties change. Most notably, the copper expands and thus the cable actually gets a little longer. This means that it takes the signal longer to travel through the cable. If the cables are not of equal length, or the heating of them is not uniform, then it means that there are errors introduced into the time of arrival of the signals at the central electronics in the RF-container. This means a degradation in performance of the instrument.

So, to mitigate these effects, the cables are buried about half a metre underground to keep their temperature more stable. There are two ways of doing this.

One is to level the field and then dig trenches for the cables. These are lined with fine sand to prevent rocks from pressing against the cables themselves, which are then laid in. A second layer of sand is then put down and the trenches backfilled and re-levelled.

The alternative is to dig down to the base level, put all the cables in place and then cover the entire region with sand and soil to bring the surface level back up to the required height, thus leaving the cables buried.